BKPF-XBLNR in clearing document

Hello,

I have some issue with substitution on Complete Document level.

I need to substitute field BKPF-XBLNR if it is empty with BGES-ZUONR from line ites. This is substituton:

Prerequisite:

( SYST-TCODE = 'FB1K' OR SYST-TCODE = 'F-44' ) AND

BKPF-BLART = 'AB'

AND

BKPF-XBLNR = ''

Substitution: Field BKPF-XBLNR is substituted by: BGES-ZUONR.

Do I missed something as nothing happened.

Thanks in advance,

Elena